1. Field of the Invention
The present invention relates to a steel sheet for automobiles which sheet is subjected to formation by pressing and the like, mainly for automobile parts. More specifically, the invention relates to a steel sheet for automobiles which is preferably used as a material for portions requiring excellent impact resistance in the event an automobile is involved in a collision, and also relates to a method of manufacturing the steel sheet.
2. Description of the Related Art
It is generally desirable to reduce the weight of an automobile body, in connection with the tendency for energy saving and environmental safeguards of the earth. It is effective to reduce the thickness of a steel sheet, while increasing the strength thereof, as a method of weight reduction.
Further, a steel sheet for automobiles is generally required to have press-formability because the steel sheet must be formed into complicated shapes.
Therefore, it is desirable that a conventional steel sheet for automobiles exhibits excellent characteristics of strength and press-formability corresponding to the strength.
However, it is insufficient for steel sheet for automobiles to be provided with only these characteristics. According to desirable design philosophy of an automobile body, the development of a steel sheet which is excellent in impact resistance for coping with collisions and the development of a steel sheet having deformation resistance when it is deformed at a high strain rate, is necessary to improve the safety of the automobile.
More specifically, one conventional method determines yield strength or tensile strength as an index of the strength of a steel sheet by the so-called static evaluation method, in which the strain rate has very low values of 10.sup.-3 -10.sup.-2 (s.sup.-1). In the design of an actual automobile body, however, strength based on the so-called dynamic evaluation method, which takes safety in collision into consideration, and accommodates deformation caused by impact having a strain rate of 10-10.sup.4 (s.sup.-1), may be more important than static strength.
Strength based on static evaluation does not always correspond to strength based on dynamic evaluation, although there is a relationship between them. As static strength increases, the dynamic/static ratio (obtained by dividing strength in dynamic deformation by static deformation) is gradually reduced. Thus, there is a problem in that when high speed deformation occurs, the advantage of increased static strength is lost.
Therefore, although the static strength of an automobile body can be improved by increasing the strength of a steel sheet, the increase of strength does not intrinsically improve the above impact resistance. In other words, there is a problem in that conventional technology cannot serve as a satisfactory solution to the problem of reducing the weight of an automobile body.
Conventionally, the quality of a steel sheet for automobiles is strengthened by a method of using a solid solution effect of matrices, achieved by the addition of substituent type elements mainly including Si, Mn and P, with steel having a structure composed of a single ferrite phase, and a method of strengthening a structure by precipitating a martensite phase, bainite phase and austenite phase in a ferrite phase.
As an example of the former method, Japanese Patent Application Laid-Open No. Sho 56(1981)-139654 proposes a steel sheet whose strength is increased in such a manner that Ti and Nb are contained in ultra-low carbon steel, to improve formability and aging property. Further strengthening components such as P and the like are contained therein in the range which does not injure formability. As an example of the latter method, Japanese Patent Application Laid-Open No. Sho 60(1985)52528 proposes a method of manufacturing a high strength thin steel sheet which improves ductility in such a manner that low carbon steel (C: 0.02-0.15 wt %) is annealed at high temperature, and a martensite phase is precipitated after the annealed steel is cooled.
However, these proposals do not consider the view point of dynamic/static ratio. In fact, a dynamic/static ratio obtained by the method proposed in Japanese Patent Application Laid-Open No. Sho 56(1981)-139654 is about 1.2, and a dynamic/static ratio obtained by the method proposed in Japanese Patent Application Laid-Open No. Sho 60(1985)-52528 is also about 1.2. Thus, it cannot be said that these steel sheets have satisfactory characteristics as a steel sheet for automobiles.
In general, in the case of mild steel, its dynamic/static ratio is about 2.0. On the other hand, in the case of high strength steel having a tensile strength (TS) of 35-40 kg/mm.sup.2, its dynamic/static ratio is about 1.2. When the dynamic/static ratio has such values, the strength ratio which is 1.7-2.0 in correspondence with a static state or a strain rate of 0.003(1/s), lowers to about 1.1-1.2 in a dynamic state in which a strain rate is 10.sup.3 (1/s). In such a situation, there has in the past been no efficient means of providing steel with high strength, and on the contrary there remains only an increase of cost resulting from the employment of means for increasing strength. Therefore, the dynamic/static ratio must be at least 1.6 to obtain a desired result, even after the increase of cost is considered.
Taking the above into consideration, a first object of the present invention is to provide a novel steel sheet for automobiles which has high strength, exhibits excellent press-formability characteristics, and at the same time exhibits excellent strength against impact resistance at a high strain rate, which objects have not been satisfactorily achieved in the past.
More specifically, the object of the present invention is to provide an impact resistant strength having a dynamic/static ratio not less than 1.6 in a conventional high strength steel sheet for automobiles.
The dynamic/static ratio is defined by dynamic yield stress/static yield stress. The dynamic yield stress means a rate of stain of 10.sup.3 (s.sup.-1), and the static yield stress means a rate of stain of 10.sup.-3 (s.sup.-1).
Further, a second object of the present invention is to provide a method of manufacturing a steel sheet having the above characteristics. Specifically, the second object of the present invention is to provide a steel sheet having the above characteristics directly by hot rolling or by subjecting a cold-rolled steel sheet to a heat treatment.
Thus, the present invention contributes to the improvement of safety in automobile bodies, and the realization of weight reduction of the automobile bodies, by providing the above steel sheet and a method of manufacturing the steel sheet.